The big Barrett fifty is too heavy to use from a small boat, because you cannot fire accurately from a rest. The boat is moving at all times, so you can only fire accurately offhand, using your splashes to adjust fire in fast semi-auto. If you put that 50 on a rest, say your cockpit combing, the barrel will just go up and down uncontrollably. And obviously, you can’t fire it offhand. Best is 5.56, because the key is to put out a lot of accurate rapid fire, from offhand (usually sitting offhand), using your splashes to adjust fire.